You should use the built-in json module, which was designed explicitly for this task:

>>> import json
>>> data = '''
... {
...   "abc": null,
...   "def": 9
... }
... '''
>>> json.loads(data)
{'def': 9, 'abc': None}
>>> type(json.loads(data))
<class 'dict'>
>>>

By the way, you should use this method even if your JSON data contains no null values. While it may work (sometimes), ast.literal_eval was designed to evaluate Python code that is represented as a string. It is simply the wrong tool to work with JSON data.

The null values become None automatically when loaded using the json module in the Python standard library.

import json
content = '["foo", {"bar":["baz", null, 1.0, 2]}]'
data = json.loads(content)
print(data)
# ['foo', {'bar': ['baz', None, 1.0, 2]}]

See official docs.

When you do json.load you are converting the JSON object into a Python object. I.e. JSON objects are converted into dicts, JSON arrays into Python lists, etc.

Python's null value is None, and that's the conversion you should expect. When you convert it back using json.dump, you'll get the same null-s back.

Consider this small snippet:

with open('in.json') as fin:
    data = json.load(fin)

print(data) # Print Python object
print(json.dumps(data, indent=4)) # Print output JSON

If the contents of in.json are:

{
    "key1": 10,
    "key2": "Comes and Goes",
    "key3": null,
    "key4": null,
    "key5": null,
    "key6": []
}

Printing the Python object results in:

{'key1': 10, 'key2': 'Comes and Goes', 'key3': None, 'key4': None, 'key5': None, 'key6': []}

And printing the json.dumps result in the exact same content as the input file:

{
    "key1": 10,
    "key2": "Comes and Goes",
    "key3": null,
    "key4": null,
    "key5": null,
    "key6": []
}

If you're using Python 2.6 or later, you can use the built-in json module:

>>> import json
>>> json.dumps([1, 2, 3, None, 4])
'[1, 2, 3, null, 4]'

See http://docs.python.org/library/json.html